The ability to produce extremely smooth, continuous surfaces on a work piece or substrate is essential to many technologies. For example, the successful fabrication of integrated circuits requires that an extremely high degree of planarity be obtained on a the surface of the workpiece (e.g., an integrated circuit or “chip”) such that successive layers of circuitry can be built upon one another while maintaining extremely small dimensions. In other areas of technology, such as fiber optics, the ability to produce extremely smooth, defect-free, contoured surfaces on the end-faces of optical fibers is a prerequisite for the formation of high-performance fiber optic connections.
Microelectronics and fiber optics polishing can be particularly difficult because the surfaces to be polished often comprise more than one type of material. Since different materials usually polish at different rates, it can be hard to obtain a continuous, smooth surface. Fiber optic ferrules, for example, typically have a rounded distal end adapted to abut against the distal end of a corresponding ferrule. The ferrule has a central bore that receives an optical fiber so that the end of the optical fiber is aligned and exposed at the apex of the rounded distal end. Accordingly, when two ferrules are coaxially aligned and positioned such that the rounded distal ends oppose each other, the apexes of the distal ends can abut, and the optical fibers can contact each other. In order to provide a smooth continuous contour, it is desirable to polish the contoured distal end of the ferrule together with the optical fiber. However, because the fiber polishes at a different rate from the material of the ferrule, it can be difficult to obtain a smooth, continuous curve in this manner.
Chemical-mechanical polishing can be used to polish substrates comprising more than one material, such as fiber optic ferrules. In order to control the global curvature of the surface, a polishing pad of an appropriate compliance is selected, such that the pad material will conform to the desired curvature when placed in contact with the fiber optic ferrule under a specific load. However, most chemical-mechanical polishing systems using a compliant polishing pad are not self-limiting, which means that the polishing system will over-polish a substrate if the polishing system is not stopped once a globally smooth surface is achieved. For example, if the natural polishing rate of the fiber optic material is less than that of the ferrule, over-polishing with a compliant pad can result in the polishing pad conforming to the optical fiber. As a result, the fiber can protrude from the end of the ferrule producing an unwanted local topography (e.g., large spherical errors). Alternatively, if the natural polishing rate of the fiber optic material exceeds that of the ferrule, over-polishing with a compliant pad can result in the fiber recessing into the ferrule. In either case, a discontinuous contour can result.
Another consideration in polishing substrates such as fiber optic ferrules is uniformity in polishing from one substrate to the next. Prior art polishing pads typically employ adhesives to join together polishing pad layers. Most adhesive-bonded pads are not separable, and the individual components of the pad, such as the polishing surface, cannot be independently replaced. As it is not economically practical to replace the entire pad after each polishing operation, the pad is typically used to polish several substrates or sets of substrates before it is replaced. However, the polishing surface of the pad changes slightly during each use as it abrades the substrate during polishing. As a result, the same polishing surface is not being used in each polishing operation, which can introduce some degree of non-uniformity in the polished surfaces. Furthermore, when layers of adhesive-bonded pads are replaced, the surface underlying the polishing surface can be damaged as a result of the adhesive tearing the underlying surface, or leaving a residue that causes the surface to be not entirely smooth. Such changes in the surface underlying the polishing surface of the polishing pad also can lead to non-uniformity in the polishing process.
Thus, there remains a need for effective polishing pads that can be used to produce extremely smooth contoured and/or planar surfaces. The invention provides such a polishing pad, as well as a method for its use. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.